Just three things
- good starter
- correct temperature
- real milk no added solids
Its the addition of solids that scews the whole process, it makes the yogurt firmer and richer while stalling out the bacteria due to PH before all/most the lactose is digested.
So most commercial yogurts can claim live culture but due to the milk solids addition most have died off...
But that keeps the masses happy with a very sweet and thick pudding..